I cannot start the game with dawnguard i have a few of mod, i try to deactivate it but nothing happen i can start the game only if i desactive dawnguard in the data. Please help me to find what can i do !

Welcome to the forums.

And to answer your question... I'd just sart a new game. With mods disabled/deleted and simply re-download Dawngaurd and your mods when your character is favorable. I had a similar problem too a while back. My saves were inaccesible after I re-downloaded an update when certain areas weren't respawning properly

Have you gone to whiterun?

I have mods and after downloading dawnguard I loaded up my last save. Nothing happened. No quest marker or anything. I was in Markarth and for some reason decided to map to Whiterun. I was instantly greeted upon my arrival to Whiterun by a male orc who's name is Durak. Seems to be working fine now.

I have the same issue. I`v played russian version (steam) and i bought dawnguard on steam, it crushes after logo, and runs only when i desactive DLC. i`v tryed to dawnload english steam version, nothing change =(

My issue was that file Skyrim. ini in my documents was set to only read option. Now works fine!